7c89d48ac0
Implements T025-T027 from TDD workflow (red-green-refactor): - T025 (red): Tests for ModbusAddress with From<RelayId> conversion - T026 (green): ModbusAddress newtype (#[repr(transparent)]) with offset mapping - T027 (red+green): HealthStatus enum with state transitions ModbusAddress wraps u16 and converts user-facing relay IDs (1-8) to Modbus addresses (0-7) at the domain boundary. HealthStatus tracks relay health with Healthy, Degraded, and Unhealthy states supporting error tracking and recovery monitoring. Ref: T025, T026, T027 (specs/001-modbus-relay-control)